In Lucian (Hermot. 54) the proverb is traced to a story of Pheidias, who, “after a look at a claw, could tell how big the whole lion, formed in proportion would be.”  A parallel Greek adage was ἐκτοῦ κρασπέδου τὸ πᾶν ὕφασμα.  Vide Leutsch., Corp. Parœmiog. Græc. I. 252.